I have a similar situation here. We have 5 dogs and 3 cats. 4 of the dogs are big heavy mutts and the 5th is a Shi Tsu (sp?) who thinks he's a Great Dane.

The bigger dogs are good natured and put up with the little one. His tactic is to bite ankles or leap into the air for necks. The cats tolerate him for awhile but will eventually send him yelping away.

Roger